Excitatory synapses in the CNS release glutamate, which acts on two types of ionotropic receptors: AMPA receptors (AMPARs) and NMDA receptors (NMDARs). Recent studies have emphasized that distinct mechanisms control synaptic expression of these two receptor classes. NMDARs are integral and relatively fixed components of the postsynaptic density (PSD), whereas AMPARs are more loosely associated. The goal of this grant is to understand the mechanisms involved in the trafficking of AMPARs at synapses. We have taken advantage of the ataxic and epileptic mutant mouse stargazer, which lacks functional AMPARs on cerebellar granule cells. Our preliminary results suggest that stargazin, the four-pass transmembrane protein disrupted in stargazer mice, binds to AMPARs and is essential for delivery of functional receptors to the plasma membrane. In addition, the C-terminal tail of stargazin can bind to PDZ domains from the postsynaptic density protein PSD-95 and related proteins. This stargazin-PDZ-95 interaction mediates synaptic targeting of AMPAR, but not their surface expression. This pathway for AMPAR trafficking may be universal as two close homologues of stargazin, gamma-3 and gamma-4, are differentially expressed throughout the brain. We now propose four specific aims to assess the mechanism and generality of stargazin-mediated trafficking of glutamate receptors. We will determine: 1) how stargazin regulates synaptic expression of AMPARs, 2) whether stargazin mediates targeting of AMPARs in interneurons, 3) whether stargazin targets kainate receptors, and 4) whether stargazin homologues traffic AMPARs. The proposed experiments will be a collaborative effort involving a combination of electrophysiological and molecular biological techniques. These studies will provide fundamental insight into mechanisms for synaptic development and short-term and long-term plasticity. Understanding mechanisms that control synaptic targeting of glutamate receptors will help clarify the role that this plasticity plays in learning and memory.